gio/gdatainputstream: use memchr() when possible

Scanning for stop chars can require looking through a considerable amount
of input data. In the case there is a single stop character, use memchr()
which can be optimized by the compiler to look at word size or greater
amounts of data at a time.

gio/gdatainputstream.c

@@ -870,6 +870,18 @@ scan_for_chars (GDataInputStream *stream,
   end = available;
   peeked = end - start;
 
+  /* For single-char case such as \0, defer to memchr which can
+   * take advantage of simd/etc.
+   */
+  if (stop_chars_len == 1)
+    {
+      const char *p = memchr (buffer, stop_chars[0], peeked);
+
+      if (p != NULL)
+        return start + (p - buffer);
+    }
+  else
+    {
       for (i = 0; checked < available && i < peeked; i++)
         {
           for (stop_char = stop_chars; stop_char != stop_end; stop_char++)
@@ -878,6 +890,7 @@ scan_for_chars (GDataInputStream *stream,
                 return (start + i);
             }
         }
+    }
 
   checked = end;